Fix Bmad collimator length on import

[jank324]

Description

Changes in interpretation of Bmad ecollimator and rcollimator in Cheetah from a lengthless Aperture element to a Drift + Aperture sequence.

Motivation and Context

• [x] I have raised an issue to propose this change (required for new features and bug fixes)

This was a bug found in the LCLS import that led to a Twiss mismatch. See #248.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation (update in the documentation)

Checklist

• [x] I have updated the changelog accordingly (required).
• [ ] My change requires a change to the documentation.
• [ ] I have updated the tests accordingly (required for a bug fix or a new feature).
• [ ] I have updated the documentation accordingly.
• [x] I have reformatted the code and checked that formatting passes (required).
• [x] I have have fixed all issues found by flake8 (required).
• [x] I have ensured that all pytest tests pass (required).
• [ ] I have run pytest on a machine with a CUDA GPU and made sure all tests pass (required).
• [x] I have checked that the documentation builds (required).

Note: We are using a maximum length of 88 characters per line.

[cr-xu]

I have only one minor concern: I guess when you have a real aperture with non-length, the beam should be smaller than some threshold over the whole length. Would it make more sense to convert it to [Aperture_start, Drift, Aperture_end]?

Edit: looking at the Bmad manual it happens only at the "exit end" by default

[jank324]

I have only one minor concern: I guess when you have a real aperture with non-length, the beam should be smaller than some threshold over the whole length. Would it make more sense to convert it to [Aperture_start, Drift, Aperture_end]?

Edit: looking at the Bmad manual it happens only at the "exit end" by default

I was thinking about the same thing. In the end, only an infinite number of apertures would be truly correct. But this quick fix worked well enough to fix the mismatch I had. We can think about adding more options or even a Collimator element at a later date if this is not enough.